In the liquid development of electrostatic-charge latent images, as in electrophotography, a substrate having a charge pattern on its surface is contacted with a liquid developer which is a suspension of colloidal toner particles in an insulating carrier liquid. The developer normally also contains a charge-control agent. The latter is an ionic compound which controls the magnitude of the charge on the toner particles and aids in maintaining a stable charge on the toner particles within the insulating carrier liquid. It may also contain an ionic dispersion stabilizer which aids in maintaining the colloidal toner particles in suspension, or a single appropriate compound may function as a charge-control agent and stabilizer.
Liquid development is used for single-stage development of electrostatic-charge patterns. It can also be used, however, in plural-stage development, for instance, in the sequential development of two or more color-separation images, or in the annotation of a developed image or in the repeated development of images on a reusable photoconductor after transfer of a developed image.
In certain plural-stage liquid development processes, a problem has arisen. The problem is especially significant in processes for the electrophotographic reproduction of multicolor images of graphic-arts quality. These processes involve exposing a photoconductor sequentially through a series of four registered color-separation transparencies with liquid development of each latent image before the next is formed. The problem is that image defects may occur when developing areas of the substrate which are background or untoned areas of a previous stage. The defect appears as a runny or smeared image in the previously untoned areas, a defect not acceptable for producing high-quality images as required in graphic arts.
It has been discovered that the described image defects in plural-stage liquid development are caused by forces or substances in the previously untoned areas which interfere with further development in those areas. Although the exact nature of those forces or substances is not certain, the present invention provides a method which reduces or eliminates the problem.